1. Field of the Invention
The present invention relates to a direct demodulation microwave frequency head allowing the reception of television or telecommunications signals retransmitted by a terrestrial satellite. This unique head, located in the vicinity of the parabolic receiving antenna, allows the simultaneous demodulation of several transmission channels from among all the channels available on several satellites. It is connected to the multiplexer of an existing UHF and VHF microwave link receiving antennas and, because of this, uses the existing UHF-VHF down lead to the television receiver, or telecommunications equipment.
2. Discussion of the Background
One of the difficulties associated with the reception of signals retransmitted by satellites is caused by the multiplicity of the forms of retransmitted signals depending on:
the orbital position of the satellites located, for France, between 20.degree. East and 40.degree. West,
their carrier band frequency between 11 GHz and 12.8 GHz
the type of polarization of the wave transmitted by the satellite which is linearly polarized in the vertical or horizontal direction, or left or right circularly polarized.
This latter characteristic is related to the technology and type of source of illumination of the antenna and is excluded from the scope of the invention but the receiving electronics must still be able to be compatible with all of the covered bandwidth which is, at present, a bandwidth of the order of 1.8 GHz.
According to the prior art, the structure of a satellite TV reception system comprises two subassemblies: an external unit, located for example on the roof of a building, and an internal unit which is often in the form of a cabinet slid under the receiver.
The function of the external unit is:
to pick up the energy transmitted by the satellite by means of an antenna,
to amplify the received signal (about -80 to -90 dBm per channel),
to transpose the received signal (bands 10.95 to 11.7, 11.7 to 12.5, 12.5 to 12.75 GHz) into an intermediate frequency band of 950 to 1700 MHz.
The function of the internal unit is:
to select the channel to be received by means of a synthesizer and a frequency converter. A surface wave filter, normally a 480 or 612 MHz filter, allows the elimination of the adjacent channels.
to demodulate the frequency modulated signal by means of:
either a standard frequency discriminator,
or a phase locked loop allowing an increase in the sensitivity threshold of the demodulator (gain of 2 to 3 dB approximately).
The essential advantage of this structure is that it minimizes the complexity of the external unit.
In the case of the simultaneous reception of several satellites, each satellite requires a specific receiving antenna. It is therefore necessary to provide, between the external units (antennas and converters) and the internal units (satellite demodulators) as many down leads as there are satellites. When two different polarizations are used simultaneously on a same satellite, it is necessary to provide a switching of the intermediate frequencies, or two antenna down leads per satellite.
In the case of the sequential reception of several satellites, it is possible to simplify the problem by means of a switching box which allows the choosing of "n" outputs from among "p" possibilities and the routing of them to the users concerned. The limit case is of a single user who wishes to sequentially receive one polarization from one satellite at a time. In this case therefore a multi-satellite antenna is necessary which is electrically aimed at the chosen satellite and which has the number of types of microwave frequency sources and of compatible converters.
But in addition, recent television sets have a video frequency picture-in-picture channel which allows the simultaneous observation of what is happening on a main program and on another program for the purpose of recording it.
It is necessary to be able to have the simultaneous availability of:
a main program
any program in a picture-in-picture mode
a program for the recording which assumes, at least, three independent satellite reception and demodulation channels. In this case, it is necessary to receive all of the signals simultaneously. It is impossible to carry out these functions with a single mono-beam antenna with motorized aiming.
By way of example, in order to receive the principal satellites available in France at the end of 1989, it would be necessary to have:
6 antennas of dimensions adapted to the different powers of the satellites,
polarizations which are either linear (H or V) or circular (C+ or C-),
receiving signals in different bands, which necessitates the use of three types of microwave frequency, heads, respectively covering the sub-bands of 10.95 to 11.7 GHz, of 11.7 to 12.5 GHz and of 12.5 to 12.75 GHz.
It is necessary to have 10 antenna down leads or a matrix of switches which allows the choosing of three programs from among 10.
It is also necessary to have at least three satellite tuners which are multi-purpose with respect to frequency excursions and with respect to sound subcarriers.
It can therefore be seen that the simultaneous reception of terrestrial UHF-VHF links and of links retransmitted by satellites results in a rather complex and expensive installation.
FIG. 1 shows an example of such an installation.
In this figure, as in all of the others except for FIGS. 8 and 9, the vertical dotted line separates the external unit, to the left of the figure, from the internal unit, to the right.
According to the prior art, a microwave and satellite reception assembly comprises two different, separate systems which join each other only at the level of the TV receiver or of the telecommunications equipment.
The "terrestrial" microwave reception circuit comprises an antenna 1, generally combined for VHF signals (40-250 MHz) and UHF signals (350-860 MHz), a multiplexer 2, and a screened coaxial cable 3, which carries an amplitude modulated signal, in a band included between 40 and 860 MHz. This cable comes down to the receiving equipment in which it feeds, by way of example, a tuner 4 which supplies a principal picture, a tuner 5 which supplies a picture within the previous one and a tuner 6 for recording by video tape recorder.
If such a conventional installation must be modified to receive several channels retransmitted by several satellites, it is necessary to add to it, by way of example:
a parabolic antenna 7, adapted to horizontally and vertically polarized signals, and two converters 8 and 9, one for each type of polarization,
a parabolic antenna 10, adapted to circularly polarized signals, and a converter 11.
For each of the converters, 8, 9, 11, a special coaxial cable 12, 13 or 14 transmits a signal which is FM modulated between 950 and 1700 MHz to the internal unit formed from a switching matrix 15 and as many FM demodulators 16, 17, 18 as there are channels transmitted by satellite which are desired to be demodulated simultaneously.
By way of example, if the antenna 7 receives 16 channels and the antenna 10 receives 4 channels, 3 cables 12, 13, 14 and 3 demodulators 16, 17, 18 are necessary for simultaneously demodulating three channels from among 20.
The disadvantage of this system is obvious, it requires a large number of converters 8, 9, 11, each of which does not have a sufficient pass band to cover the bandwidth covered by all of the satellites, and it requires a large number of link cables 12, 13, 14.
With regard to the link cables, it is therefore necessary to install new cables, in addition to the cable 3, which carry signals which are FM modulated at 0.95-1.75 GHz. Because of this, the installation constraints result in high costs.
With regard to the converters, FIG. 2 shows why their pass bandwidth is insufficient. The converters presently used comprise a low noise amplifier 19, which receives the signal coming from the parabolic antenna 7, for example, then a mixer 20 and an output amplifier 21. The mixer 20 also receives the signal coming from a local oscillator 22 which, for practical and economic reasons, is generally a dielectric resonator oscillator, or DRO (Dielectric Resonator Oscillator).
In order for this converter to be multi-band, and adapted to the reception of several satellites, it is necessary for it to have several single frequency or switchable local oscillators 22. At present, the most common solution is a local oscillator having two switchable frequencies, which allows the receiving of only two sub-bands per converter channel.
This solution is insufficient with regard to the number of satellites in orbit, and which are to come, and with regard to the number of channels available for TV retransmission and telecommunications.